The present invention relates to the vapor hydrogen peroxide arts. It finds particular application in the sensing of vapor hydrogen peroxide concentrations, as well as in the control of vapor hydrogen peroxide sterilization and other processing systems.
Typically, hydrogen peroxide vapor sterilization systems include a heater for vaporizing an aqueous hydrogen peroxide solution. The hydrogen peroxide and water vapor are carried in and through a sterilization or other treatment chamber. In the chamber, the hydrogen peroxide vapor kills microorganisms in an oxidizing reaction which converts the hydrogen peroxide vapor into water vapor. The water vapor and remaining hydrogen peroxide vapor are typically exhausted from the chamber. Because hydrogen peroxide breaks down quickly in sunlight, the vapor can be discharged to the atmosphere. Alternately, the hydrogen peroxide vapor can be converted into water vapor and the carrier air dried and recirculated to the vaporizer in a closed loop.
Typically, the aqueous hydrogen peroxide is fed to the vaporizer at a rate intended to maintain a preselected minimum hydrogen peroxide concentration in the chamber, but not so fast that the vapor in the chamber becomes saturated. Condensation will deplete the concentration of the hydrogen peroxide in the vapor (the condensate being greatly enriched in hydrogen peroxide), reducing the overall efficacy of the process. The condensate (being concentrated hydrogen peroxide) may also cause significant material compatibility issues. However, other than setting cycle parameters to avoid oversaturation of the vapor, the saturation of the process has not typically been monitored or controlled. The temperature in the chamber is typically held at a preselected temperature that is selected in accordance with the temperature compatibility of the products being sterilized in the chamber. The selected temperature is selected sufficiently high that the hydrogen peroxide vapor reacts effectively with microorganisms, but not so high that the hydrogen peroxide breaks down into oxygen and water vapor at too high a rate to maintain the selected hydrogen peroxide concentration in the chamber.
Various techniques have been proposed for monitoring hydrogen peroxide vapor concentration. These include electrochemical methods such as catalytic gates, amperometric measurements, and potentiometric measurements. However, electrochemical measurement techniques have various drawbacks. Typically, they rely on mass transfer across a gas/liquid or gas/solid interface. This results in longer time constants and a slow response of the control system. The output of electrochemical sensors is sensitive to the amount of gas movement proximate to the sensor. Electrochemical methods are not species specific. Interfering species cannot typically be separated. Calibration of electrochemical sensors is complex and typically requires return of the sensor to the manufacturer for factory calibration. Further, electrochemical techniques are temperature and pressure sensitive. The need for temperature compensation complicates calibration. When the sterilization chamber is held in the vacuum range, the vacuum significantly complicates and can even defeat concentration measurements with electrochemical methods. Electrochemical sensors typically do not measure the concentration of water vapor requiring use of a humidity sensor if saturation is to be determined.
Concentrations of water and hydrogen peroxide vapor can also be determined with spectroscopic methods. Hydrogen peroxide and water vapor both have a multiplicity of spectral lines at characteristic frequencies. In the infrared ranges, the spectral lines are so dense that they are generally viewed as a continuum, hydrogen peroxide and water vapor each having a characteristic curve or frequency absorption spectrum.
Hydrogen peroxide has a spectrum in the ultraviolet range. Ultraviolet light is not strongly absorbed by hydrogen peroxide, hence a weak signal is provided. Also, ultraviolet light degrades hydrogen peroxide breaking it down into water vapor and oxygen. Further, although hydrogen peroxide concentrations can be measured with ultraviolet light, there is typically no measurement of water vapor concentration. Without measuring the water vapor, the percent saturation cannot be determined.
Hydrogen peroxide and water vapor both have spectral peaks in the near infrared range. Although there may be spectral lines that are unique to water vapor or hydrogen peroxide vapor, the lines are closer together than the spectral differentiation of commonly available infrared sensors. When viewed as peaks or continuums, the hydrogen peroxide and water vapor peaks overlap significantly. Relatively complex calculations are needed to determination the concentrations of hydrogen peroxide and water vapor, individually, from the partially overlapping peaks. Moreover, there is a relatively weak energy transfer in the near infrared range. Thus, the output signals tend to be relatively weak. To eliminate noise problems in the weak signals, relatively expensive, more noise-free hardware is commonly utilized. Further, to improve the signal-to-noise ratio, a relatively long path length for the near infrared light through the vapor is utilized, often on the order of about 25 cm.
Prior techniques for controlling hydrogen peroxide vapor concentration have lacked sufficient credibility to be used alone to show that sterilization or high level disinfection in the chamber has been achieved. Rather, chemical indicators or biological indicators are commonly placed in the chamber. Chemical indicators undergo a color or other physical change in response to contact with peroxide vapor over time. However, because the chemical indicators typically only measure hydrogen peroxide concentration integrated over time, they are again only an indicator that sterilization or high level disinfection has been achieved and not considered proof. To prove that sterilization or disinfection has been achieved, biological indicators are typically utilized. Test spores in the biological indicators are exposed to the vapor in the chamber. The spores are then incubated for several days to see if any grow. Although highly reliable, biological indicators take several days to read. Typically, the sterilized goods are held in inventory for several extra days after sterilization until the biological indicator tests results are returned.
The present invention overcomes the above-referenced problems and others.